Band pass amplifier



Sept. l2, 1933. Y B, D H TELLEGEN 1,926,125

BAND PAss AMPLIFIER Filed July 7, 1930 INVENTOR ERNARDus .H. TELLEGENATTOR N EY `fPatenteci Sept. 172, 1933 STATES AmrsNr BAND PASSAMPLIFIER.

Application July 7, 1930, Serial No. 465,972, and in the NetherlandsAugust 3, 1929 Claims. (Cl. 178-41) The present invention relates toband pass filters which are particularly adapted to be used in wirelessreceiving sets.

It is known that when transmitting telephony 5 a definite band widthmust be strongly received, while the frequencies lying immediatelyoutside of this band should be suppressed as much as possible. Arectangular resonance curve is often considered to be ideal. Thereby, aband, whose width corresponds to the range of speech or musicfrequencies, is received with a constant intensity, while the higher andlower frequencies are not received at all. y

Such a rectangular resonance curve can, however, notrbe obtained withthe aid of available means. In fact, all of the curves indicating thecurrent intensity, or the voltage, occurring in anirnpedance independence of the frequency have a more or less smooth shape, and it hasbeen almost impossible to design a combination such that the frequenciesbelow, or above, a certain limit are completely excluded. Although Vbyusing coupled circuits this rectangular resonance curve may beapproximately more or less closely, it appears that the frequencieslying immediately outside of the range which is desired to be received,are still allowed to pass at an intensity which is too high to assure asufficient elimination of these undesired frequencies.

This invention has for its main object the provision of a new method bymeans of which the shape of the resonance curve which has beenconsidered hereinbefore as being ideal, is more closely approximatedthan it has been possible by using the systems 1hitherto known. Theprinciple of the invention involves the combination of impedances sochosen, that immediately before, and immediately after, the range whichmust be strongly received one or more minima occur in the totalresonance curve. Although in these circumstances the resonance curvewill slightly rise over a certain distance as well in the direction ofthe higher side as in the direction of the lower side, it has appearedthat the advantages obtained by applying the aforementioned principle,are retained thereby.

The novel features which I believe to be characteristic of my inventionare set forth in par- ,50 ticularity in the appended claims, theinvention itself, however, as to both its organization and method ofoperation will best be understood by reference to the followingdescription taken in connection with the drawing in which I haveindicated diagrammatically several circuit organizations whereby lmyinvention may be carried into effect.

The invention will be more clearly understood by reference to theaccompanying drawing, representing in Figs. l to 5 inclusive variousembodi- 60 ments of band pass filters according to the in- Vention.

In Fig. l the band pass filter consistsof a stopping circuit L1, C1,connected in series to an inductance L3. This inductance Ls ismagnetically coupled with an inductance Li whose terminais are connectedacross a series connected combination consisting of an inductance L2 anda capacity C2. Let it be assumed that a Voltage be applied to the pointsa and b, and that the voltage is collected at the points c and d, thenit can be shown that the proportion between the output voltage and theinput voltage becomes equal to zero at the frequencies:

This proportion is, oi course, only zero if the damping caused by ohmicresistances is not taken into consideration. However, it will beobvious, that even in presence of ohmic resistances, minima of theresonance curve may occur at the frequencies w1 and wz. When the dampingincreases, the minima may become less sharp, or even completelydisappear, without 'the effect obtained being entirely lost.

The frequencies w1 and m2 are on either side of the frequency band to betransmitted. if w1 is the lowest frequency the resonance curve will,iirst, rise steeply immediately subsequent to w1, and, then, show aplane shape for a certain distance. Thereupon, it will steeply descendimmediately preceding a2, and slowly rise subsequent to wz. At any rate,the permeability in this case will remain small with respect to thatwhich is encountered by the desired frequency. By a suit- 100 ablechoice of impedances the peak of the resonance curve will substantiallyextend parallel with respect to the axis of the frequency.

A still better result may be obtained by applying the diagram shown inFig. 2, comprising a circuit L1, C1, C3 which, by means of the capacityC4, is capacitively coupled with the circuit L5, C5, the latter being inturn magnetically coupled with the circuit Le, C2, Lz. When a current issupplied at the points c and b, and the voltage is 110 collected at thepoints c and d, the ratio between plates.

heterodyne receivng sets.

For these frequencies minima will occur again,

and

while between these frequencies the resonance curve has the same shapeas that belonging to the system shown in Fig. l. The supply of lacurrent between the points a and b may preferably be effected byinserting the filter in the anode circuit of a triode, or of a tetrode,having a high internal resistance in the manner as indicated in Fig. 2.The connection with the nextftriode may be established by connecting thepoints c and d respectively to the grid and the cathode of said nexttriode.

lIn Fig. 3, a system is shown which is identical to the systemillustrated in Fig. 2. In this case the capacitive coupling which iseffected by the capacity of Ci, indicated in Fig. 2, is replaced by acoupling established by means of a thermionic device 2. The operation ofthe circuit remains, however, the same; only the dependency between bothparts of the filter included in the'system shown in Fig. 3 is a littleless strong vas it is the case in the system represented in Fig. 2. Infact, the value of the capacity Ci in Fig. 4., also, determines theposition of the resonance frequencies While the triode 2 shown in Fig. 3only Very slightly acts upon the resonance frequencies, so that -theproperties of the system shown in Fig. 3 may easily be derived from theproperties of both parts individually which are coupled together bymeans of the triode. With the lters shown in Figs. 2 and 3 the feedingof the plates may be effected by inserting choke coils and a source ofcurrent between the points a and b.

Figs. 4 and 5 present some further filters in which the invention isembodied and whereby no separate choke coils are needed for feeding theAlso with these lters there occur minima in the frequencies which aredetermined by Li, Ci and L2, C2. y

By properly proportioning the variable cornponents of thesystemsdescribed hereinbefore the whole filter vcircuit may be rendered tunableso that `the width ofthe frequency band which is allowed to pass remainssubstantially constant. In some cases the filter circuit-need not betunable, especially in case it is applied in super- In the latter casethe filter may be made so as to constitute an intermediate frequencyfilter. manent lter may be applied when transmitting high frequencytelephony along lines; since thereby only a number of fixed wave lengthsare used each receiver may be given his own filter.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications in the circuitarrangements, as well as in the apparatus employed, may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is: y

1. In an electric transmission system, a source of high frequencyenergy, means for utilizing the energy, means for coupling the source tosaid utilizing means, an oscillatory circuit connected Furthermore, aper-1' between said source and said coupling, an oscillatory circuitconnected in shunt with said coupling and disposed between the latterand the utilizing means, said first oscillatory circuit being resonantVto one limiting frequency of a desired band of frequencies, and thesecond oscillatory circuit being resonant to the other limitingfrequency of said band whereby the resonance curve of the system risessteeply at each limiting frequency, has a plane shape for a certaindistance between the steep points and rises slowly outside the limitingfrequencies.

2. In an electric transmission system, a source of high frequencyenergy, means for utilizing the energy, means for coupling the source tosaid utilizing means, an oscillatory circuit connected between saidsource and said coupling, an oscillatory circuit connected in shunt withsaid coupling and disposed between the latter and the utilizing means,said first oscillatory circuit being resonant to one limiting frequencyof a desired band of frequencies, and the second oscillatory circuitbeing resonant to the other limiting frequency of said band whereby theresonance curve of the system rises steeply at each limiting frequency,has a plane shape for a certain distance between the steep points andrises slowly outside the limiting frequencies, said first limitingfrequency being the lowest frequency of the said band, and the secondlimiting frequency being the highest frequency of the band.

3. In an electric transmission system, a source of high frequencyenergy, means for utilizing the energy, means for coupling the source tosaid utilizing means, an oscillatory circuit connected in series betweensaid source and one side of said coupling, an oscillatory circuitconnected in shunt with said coupling and disposed between the latterand the utilizing means said first oscillatory circuit being resonant toone limiting frequency of a desired band of frequencies, and the secondoscillatory circuit being resonant to the other limiting frequency ofsaid band whereby the resonance curve of the system rises steeply ateach limiting frequency, has a plane shape for a certain distancebetween the steep points and rises slowly outside the limitingfrequencies.

4. In an electric transmission system, a source of high frequencyenergy, means for utilizing the energy, means for coupling the source tosaid utilizing means, an oscillatory circuit connected between saidsource and said coupling, an oscillatory circuit connected in shunt withsaid coupling and disposed between the latter and the utilizing means,said first oscillatory circuit being resonant to one limiting frequencyof a desired band of frequencies, and the second oscillatory circuitbeing resonant to the other limiting frequency of said band whereby theresonance curve of the system rises steeply at each limiting frequency,has a plane shape for a certain distance between the steep points andrises slowly outside the limiting frequencies, said source comprising anelectron discharge tube having a high internal resistance, said couplingmeans consisting of a transformer.

5. In an electric transmission system, a source of high frequencyenergy, means for utilizing the energy, means for coupling the source tosaid utilizing means, an oscillatory circuit connected between saidsource and said coupling, an oscillatory circuit connected in shuntwithv said coupling and disposed between the latter and the utilizingmeans, said first oscillatory circuit besource" cbrnp'ri'sing' anelectron discharge tube' having a high internal resistance, saidcoupling means consisting of a transformer and additional capacitivemeans coupling said rst oscillatory circuit. and said. transformer.

BERNARDUS DOMINrUs' HUBERTUS TELLEGEN;

